Curved or cylindrical permanent magnets having a radially oriented magnetic field are commonly used in electrical motors and generators, in eddy current devices and in magnetic bearings. The radial orientation of the field permits the full force of the field strength to be directed towards the center of the circle, and this feature is highly desirable in such applications. Typically, these magnets are formed from rare earth-transition metal compounds because such magnets have magnetic energy products markedly higher than those of conventional permanent magnet compounds. Also, in DC motors, the size and weight of the motors equipped with such magnets can be substantially reduced over conventional DC motors which require heavy copper windings or bulky iron poles or ferrite magnets.
In the past, rare earth-cobalt permanent magnets have been formed by a process which involves alignment and die pressing of a powder in a magnetic field to form an aligned compact and subsequent sintering of this compact at temperatures greater than 1100.degree. C. In such magnets, densification to only 93% to 95% of the theoretical maximum is possible, and further densification results in rapid crystal growth which leads to lowered coercivity. This low coercivity is suspected to result from a reasonably large particle size and a high oxygen content. If smaller particle sizes are used, the oxygen content of the magnet is increased because of contamination of the powder by exposure to air, even at room temperatures. Larger particle sizes cannot be used in a sintering process because of inadequate sintering that results from their use. Since the oxygen levels of conventional sintered material are quite high, typically 0.5 to 1.0 weight percent, the coercivity retaining ability of the material is reduced at intermediate temperatures. Examples of magnets formed by this process are described in U.S. Pat. Nos. 3,665,463; 3,919,003; 4,002,508; and 4,076,561. Rare earth-transition metal magnets may also be formed by hot isostatic pressing, as described in U.S. Pat. No. 3,615,915.
Many methods have been tried in the past for forming radially oriented magnets, with few of them being particularly successful. One practice has been to grind into a thin curved shape flat magnets having magnetic domains aligned in a perpendicular direction with respect to their flat surface. Such grinding is time-consuming and wasteful of relatively expensive rare earth-transition metal materials. Moreover, the direction of magnetic alignment of the resulting magnets is not uniformly radial and is not optimal for the shape of the device in which it is to serve. Another approach has been to deform a flat, sintered slab magnet into a curved shape, as shown for example in U.S. Pat. No. 3,864,808. The flat predensified magnets are heated to a temperature below the sintering temperature of the magnet but at which plastic deformation takes place under pressure exerted by a forming die resting on top of the magnet. However, the magnets must be deformed slowly to prevent them from breaking or distorting and such a process is only effective for shaping very thin, small magnets. Other approaches have been to radially magnetize randomly oriented or isotropic magnets, but the energy product of these magnets is only one-fourth of the theoretical maximum and thus the magnetic field strength is drastically reduced. In other applications, a large number of rectangular, line oriented magnets is assembled along the circumference of a circle, thus providing an approximation of a radially oriented field. The larger the number of magnets used the more closely true radial orientation is approximated, but the fabrication process is highly labor intensive and thus the cost is high. Additionally, the field can never be totally radially oriented since only the central portion of each rectangle is truly radially aligned. Arc segments in the green compacted state with small included angles and good radial orientation may be produced by conventional pressing, but such segments tend to loose their geometry during sintering. Radial arc segments of up to 114.degree. included angle, with lengths of up to about two inches and thin walls have been produced by die pressing and sintering, as described in U.S. Pat. No. 4,144,060. However, this method is not capable of producing full circle radially oriented magnets because of distortion during sintering. Radial arc segments have also been produced by hot isostatic pressing in a step-wise process described in U.S. Pat. Nos. 4,104,787 and 4,123,297. However, the methods described in these patents do not provide the full circle geometry desired for some applications, nor do they permit the formation of cylindrical magnets of any axial length. In addition, the field produced by these magnets includes fringing field distortions.